1. Technical Field
The present disclosure relates in general to methods for drilling wells in subterranean formations, and more particularly to methods of using resistivity data to identify a top of a formation, while the drill bit advances toward but does not penetrate the formation, in order to obtain a whole core from the formation.
2. Background Art
Formation resistivity measurements are commonly made in oil and gas wells and then used to make decisions about the presence of hydrocarbons, the magnitude of pore pressure, the correlation to formations observed in offset wells, the salinity of formation fluids, porosity of formations, and the presence of permeability. FIG. 1 illustrates graphically the prior art concept of measuring resistivity as a function of depth, showing a typical decrease in resistivity at a depth where increased geopressure (pore pressure) exists (from Eaton, “The Effect of Overburden Stress on Geopressure Prediction From Well Logs”, SPE 3719 (1972)). In shale rocks, resistivity data points diverge from the normal trend toward lower resistivity values, owing to high porosity, overpressured formations.
Existing techniques to measure resistivity are made after the bit penetrates the formation using either electric line logging methods or logging while drilling methods. In either case the formation of interest has already been exposed to the well in order to make the resistivity measurement. This exposure presents problems, including the fact that the condition of the borehole itself and surrounding disturbed formation will have an effect on the very resistivity values being sought, as noted by Hottman et al., “Estimation of Formation Pressures From Log-Derived Shale Properties”, SPE 1110 (1965).
Banning et al. discuss a theoretical application of time-domain electromagnetics (TEM) in a borehole-conveyed logging tool. Banning et al., “Imaging of a subsurface conductivity distribution using a time-domain electromagnetic borehole conveyed logging tool”, Society of Exploration Geophysicists, San Antonio Annual Meeting (2007). See also Published U.S. Patent applications Nos. 2005/0092487; 2005/0093546; 2006/003857; 2006/0055411; 2006/0061363; 2006/0061364, and U.S. Pat. No. 6,856,909. Banning et al. state that, theoretically, such a tool may be used to image the conductivity distribution around and ahead of the drill bit at comparatively large distances from the borehole. However, Banning et al. do not disclose or suggest use of resistivity measurements in front of a drilling bit to detect a top of a region of interest of a formation and make core drilling decisions to obtain a whole core before the bit exposes the formation to the drilled wellbore.
It is known in wellbore planning and drilling operations to study data from offset wells to develop and validate geomechanical stress models, and adjust casing points and mud weights to meet well challenges. See for example Brehm et al., “Pre-drill Planning Saves Money”, E & P, May 2005. An offset well is an existing wellbore close to a proposed well that provides information for planning the proposed well. In planning development wells, there are usually numerous offsets, so a great deal is known about the subsurface geology and pressure regimes.
Obtaining samples of formation rock is a common task in drilling operations. Samples, referred to as cores, are usually obtained using a core bit. A core bit is a drilling tool with a hole through the center that removes sediment rock and allows the core pedestal to pass through the bit and into the core barrel. Different coring systems and bits are employed to obtain continuous cores depending on the rock type. Once a coring system is selected based on the expected lithology, the engineer determines which type of core bit to use. As coring conditions change, the coring bit can be changed in an attempt to improve the recovery and rate of penetration with that coring system. The type of bit used depends on the expected lithology and past bit performance in the area or in a similar lithology.
Most coring systems in use today are not designed to be used to drill the formations overlying those just above the desired coring point. The core receiving area within the drill string necessitates that conventional bottom hole assemblies (BHA) be used for making measurements while drilling (MWD), logging while drilling (LWD) or rotary steering systems (RSS) be pushed back up the drill string which can significantly reduce the effectiveness or negate the purpose of them being in the drill string. Also the core barrels can only store limited amount of core, so coring assemblies are usually picked up just at the point the core acquisition is desired to maximize the amount of core that can be acquired.
Acquired core can be affected by exposure to drilling mud. The effects may reduce the value of the core in evaluating the formation being investigated. Drilling mud additives may be used in the drilling fluid to minimize effects on the core, or to identify the influence the drilling fluids may have had upon the core, or how they may have altered the core's properties. The additives can be expensive and are therefore not usually added until immediately before coring. Knowing when one is about to expose the targeted formation would allow these to be added to the mud before the mud affects the formation negatively.
To avoid or reduce these undesirable consequences, it would be advantageous if resistivity measurements in front of the coring bit could be used to detect a top of a of a formation or region of a formation and make core drilling decisions to obtain a whole core before the bit exposes the formation. In addition, there may be safety and economic advantages gained if a resistivity measurement could be made before the formation was actually exposed to the well. The methods and apparatus of the present disclosure are directed to these needs.